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Protostar to Main Sequence
• A protostar contracts and heats until the coretemperature is sufficient for hydrogen fusion.
• Contraction ends when energy released byhydrogen fusion balances energy radiated from thesurface.
• It takes 50 million years for a star like the Sun(less time for more massive stars).
Summary of Star Birth
1. Gravity causes gas cloud toshrink and fragment
2. Core of shrinking cloudheats up
3. When core gets hot enough,fusion begins and stops theshrinking
4. New star achieves long-lasting state of balance
Upper Limit on a Star’s Mass• Photons exert a
slight amount ofpressure when theystrike matter.
• Very massive starsare so luminous thatthe collectivepressure of photonsdrives their matterinto space.
Upper Limit on a Star’s Mass• Models of stars
suggest thatradiation pressurelimits how massivea star can be withoutblowing itself apart.
• Observations havenot found stars moremassive than about150MSun.
Lower Limit on a Star’s Mass
• Fusion will not begin in a contracting cloud if somesort of force stops contraction before the coretemperature rises above 107 K.
• Thermal pressure cannot stop contraction because thestar is constantly losing thermal energy from itssurface through radiation.
• Is there another form of pressure that can stopcontraction?
Degeneracy Pressure:
Laws of quantum mechanics prohibit two electronsfrom occupying the same state in the same place.
Thermal Pressure:
Depends on heat content
The main form of pressurein most stars
Degeneracy Pressure:
Particles can’t be in samestate in same place
Doesn’t depend on heatcontent
Brown Dwarfs• Degeneracy pressure
halts the contractionof objects with<0.08MSun beforethe core temperaturebecomes hot enoughfor fusion.
• Starlike objects notmassive enough tostart fusion arebrown dwarfs.
Brown Dwarfs• A brown dwarf
emits infrared lightbecause of heat leftover fromcontraction.
• Its luminositygradually declineswith time as it losesthermal energy.
Temperature
Luminosity
Stars moremassivethan150MSunwould blowapart.
Stars lessmassivethan0.08MSuncan’tsustainfusion.
A starremains onthe mainsequence aslong as it canfuse hydrogeninto helium inits core.
Main-Sequence Lifetimes and Stellar Masses
Thought Question
What happens when a star can no longer fusehydrogen to helium in its core?
A. Core cools offB. Core shrinks and heats upC. Core expands and heats upD. Helium fusion immediately begins
Life Track After Main Sequence• Observations of star
clusters show that astar becomes larger,redder, and moreluminous after itstime on the mainsequence is over.
Broken Thermostat• As the core contracts,
H begins fusing to Hein a shell around thecore.
• Luminosity increasesbecause the corethermostat isbroken—the increasingfusion rate in the shelldoes not stop the corefrom contracting.
Helium fusion does not begin right away because itrequires higher temperatures than hydrogen fusion—largercharge leads to greater repulsion.
The fusion of two helium nuclei doesn’t work, so heliumfusion must combine three He nuclei to make carbon.
Thought Question
What happens in a low-mass star when core temperature risesenough for helium fusion to begin?
A. Helium fusion slowly starts up.B. Hydrogen fusion stops.C. Helium fusion rises very sharply.
(Hint: Degeneracy pressure is the main form of pressurein the inert helium core.)
Helium Flash
• The thermostat is broken in a low-mass red giantbecause degeneracy pressure supports the core.
• The core temperature rises rapidly when heliumfusion begins.
• The helium fusion rate skyrockets until thermalpressure takes over and expands the core again.
Life Track After Helium Flash• Models show that a
red giant shouldshrink and becomeless luminous afterhelium fusionbegins in the core.
Life Track After Helium Flash• Observations of star
clusters agree withthose models.
• Helium-burningstars are found in ahorizontal branchon the H-R diagram.
Combiningmodels ofstars ofsimilar agebut differentmass helpsus to age-date starclusters.
Using the H-R Diagram to Determine the Age of a Star Cluster
Thought Question
What happens when the star’s core runs out of helium?
A. The star explodes.B. Carbon fusion begins.C. The core cools off.D. Helium fuses in a shell around the core.
Double-Shell Burning
• After core helium fusion stops, He fuses intocarbon in a shell around the carbon core, and Hfuses to He in a shell around the helium layer.
• This double-shell-burning stage never reachesequilibrium—the fusion rate periodically spikesupward in a series of thermal pulses.
• With each spike, convection dredges carbon upfrom the core and transports it to the surface.
Planetary Nebulae• Double-shell
burning ends with apulse that ejects theH and He into spaceas a planetarynebula.
• The core left behindbecomes a whitedwarf.
Planetary Nebulae• Double-shell
burning ends with apulse that ejects theH and He into spaceas a planetarynebula.
• The core left behindbecomes a whitedwarf.
Planetary Nebulae• Double-shell
burning ends with apulse that ejects theH and He into spaceas a planetarynebula.
• The core left behindbecomes a whitedwarf.
Planetary Nebulae• Double-shell
burning ends with apulse that ejects theH and He into spaceas a planetarynebula.
• The core left behindbecomes a whitedwarf.
End of Fusion
• Fusion progresses no further in a low-mass starbecause the core temperature never grows hotenough for fusion of heavier elements (some Hefuses to C to make oxygen).
• Degeneracy pressure supports the white dwarfagainst gravity.
CNO Cycle• High-mass main-
sequence stars fuseH to He at a higherrate using carbon,nitrogen, andoxygen as catalysts.
• A greater coretemperature enablesH nuclei toovercome greaterrepulsion.
Life Stages of High-Mass Stars
• Late life stages of high-mass stars are similar tothose of low-mass stars:—Hydrogen core fusion (main sequence)—Hydrogen shell burning (supergiant)—Helium core fusion (supergiant)
Advanced Nuclear Burning
• Core temperatures in stars with >8MSunallow fusion of elements as heavy as iron.
Multiple-Shell Burning• Advanced nuclear
burning proceeds ina series of nestedshells.
The Death Sequence of a High-Mass Star
Iron is a deadend for fusionbecause nuclearreactionsinvolving irondo not releaseenergy.
(Fe has lowestmass pernuclearparticle.)
Iron builds upin the core untildegeneracypressure can nolonger resistgravity.
The core thensuddenlycollapses,creating asupernovaexplosion.
The Death Sequence of a High-Mass Star
Supernova Explosion• Core degeneracy
pressure goes awaybecause electronscombine withprotons, makingneutrons andneutrinos.
• Neutrons collapse tothe center, forming aneutron star.
Energy and neutrons released in a supernova explosion enableelements heavier than iron to form, including Au and U.
Supernova Remnant• Energy released by
the collapse of thecore drives outerlayers into space.
• The Crab Nebula isthe remnant of thesupernova seen inA.D. 1054.
Multiwavelength Crab Nebula
Role of Mass
• A star’s mass determines its entire life storybecause it determines its core temperature.
• High-mass stars have short lives, eventuallybecoming hot enough to make iron, and end insupernova explosions.
• Low-mass stars have long lives, never become hotenough to fuse carbon nuclei, and end as whitedwarfs.
Low-Mass Star Summary
1. Main Sequence: H fuses to Hein core
2. Red Giant: H fuses to He inshell around He core
3. Helium Core Burning:He fuses to C in core while Hfuses to He in shell
4. Double-Shell Burning:H and He both fuse in shells
5. Planetary Nebula: leaves whitedwarf behindNot to scale!
Reasons for Life Stages
• Core shrinks and heats until it’shot enough for fusion
• Nuclei with larger chargerequire higher temperature forfusion
• Core thermostat is brokenwhile core is not hot enoughfor fusion (shell burning)
• Core fusion can’t happen ifdegeneracy pressure keeps corefrom shrinking
Not to scale!
Life Stages of High-Mass Star
1. Main Sequence: H fuses to Hein core
2. Red Supergiant: H fuses to Hein shell around He core
3. Helium Core Burning:He fuses to C in core while Hfuses to He in shell
4. Multiple-Shell Burning:many elements fuse in shells
5. Supernova leaves neutron starbehindNot to scale!
Thought Question
The binary star Algol consists of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant star.
What’s strange about this pairing?
How did it come about?